To an increasing extent, touch-sensitive panels are being used for providing input data to computers, electronic measurement and test equipment, gaming devices, etc. The panel may be provided with a graphical user interface (GUI) for a user to interact with using e.g. a pointer, stylus or one or more fingers. The GUI may be fixed or dynamic. A fixed GUI may e.g. be in the form of printed matter placed over, under or inside the panel. A dynamic GUI can be provided by a display screen integrated with, or placed underneath, the panel or by an image being projected onto the panel by a projector.
There are numerous known techniques for providing touch sensitivity to the panel, e.g. by using cameras to capture light scattered off the point(s) of touch on the panel, or by incorporating resistive wire grids, capacitive sensors, strain gauges, etc into the panel.
US2004/0252091 discloses an alternative technique which is based on frustrated total internal reflection (FTIR). Light sheets are coupled into a panel to propagate inside the panel by total internal reflection. When an object comes into contact with a surface of the panel, two or more light sheets will be locally attenuated at the point of touch.
Arrays of light sensors are located around the perimeter of the panel to detect the received light for each light sheet. A coarse tomographic reconstruction of the light field across the panel surface is then created by geometrically back-tracing and triangulating all attenuations observed in the received light. This is stated to result in data regarding the position and size of each contact area.
US2009/0153519 discloses a panel capable of conducting signals. A “tomograph” is positioned adjacent the panel with signal flow ports arrayed around the border of the panel at discrete locations. Signals (b) measured at the signal flow ports are tomographically processed to generate a two-dimensional representation (x) of the conductivity on the panel, whereby touching objects on the panel surface can be detected. The presented technique for tomographic reconstruction is based on a linear model of the tomographic system, Ax=b. The system matrix A is calculated at factory, and its pseudo inverse A−1 is calculated using Truncated SVD algorithms and operated on the measured signals to yield the two-dimensional (2D) representation of the conductivity: x=A−1b. The suggested method is both demanding in the term of processing and lacks suppression of high frequency components, possibly leading to much noise in the 2D representation.
US2009/0153519 also makes a general reference to Computer Tomography (CT). CT methods are well-known imaging methods which have been developed for medical purposes. CT methods employ digital geometry processing to reconstruct an image of the inside of an object based on a large series of projection measurements through the object. Various CT methods have been developed to enable efficient processing and/or precise image reconstruction, e.g. Filtered Back Projection, ART, SART, etc. Often, the projection measurements are carried out in accordance with a standard geometry which is given by the CT method. Clearly, it would be desirable to capitalize on existing CT methods for reconstructing the 2D distribution of an energy-related parameter (light, conductivity, etc) across a touch surface based on a set of projection measurements.